Alkyd resins have been widely used as coating materials over the years due to their many desirable physical properties and low material cost. Especially, architectural varnishes and enamels which require high gloss have predominately adopted the solvent based alkyds. However, rising health and environmental concerns over organic compound emission from solvent based paints has resulted in strict regulation on the amount of volatile organic compounds (VOC) emitted from paint after being applied.
In order to comply with such challenging regulations, many research efforts have been exerted toward developing water-borne and high solids coatings which employ far lower amounts of organic solvents than conventional coatings.
Water-borne coatings which use water as a dispersing medium are mainly latexes and water-dispersible resins. Although showing an impressive success in replacing organic solvent coatings in many applications, water-borne coatings have not been able to satisfy the coating industry's needs completely because of their inherent problems such as insufficient gloss, water sensitivity, and the difficulty of controlling water evaporation after application.
As a result, other high solids systems, which employ less organic solvent than the conventional ones, have also been gaining a lot of attention. Since viscosity usually increases dramatically with a decrease in amount of solvent, it has often been necessary for high solids systems to use lower resin molecular weights to maintain a workable paint viscosity. Due to possible adverse effects on coating properties caused by low molecular weight, additional functionality, which may build up the coating molecular weight after application is usually required for such high solids systems.
It is well known that alkyds form a crosslinked structure through an oxidation of unsaturated fatty acid at ambient conditions after being applied on a substrate. In order to offset lower molecular weights than conventional resins, high solids alkyds require an increased level of unsaturated fatty acids, which may be considered as the functional component in alkyd, to ensure enough crosslinking to achieve acceptable coating properties. One of the major drawbacks in such high solids alkyds is severe yellowing development in the cured coatings due to the increased level of unsaturated fatty acids. Oxidation of fatty acid double bonds is accompanied by development of yellow color bodies.
In order to overcome such problems, the polyhydroxy based allyl ethers have been incorporated into alkyds to replace a part of or all the fatty acids. Allyl ether compounds undergo oxidation as does unsaturated fatty acid; however, such oxidation does not cause yellowing in the cured coatings. However, the synthesis of polyhydroxy based allyl ethers usually results in a statistical distribution of allyl ether functionality in polyhydroxy compounds due to the reaction between monofunctional allyl compound and multi-functional hydroxy compound. This makes it quite difficult to design a low-VOC resin since the heterogeneity in hydroxy functionality will broaden molecular weight distribution. Furthermore, since the two starting raw materials, allyl alcohol and polyhydroxy compound, react reluctantly with each other, the need of special processes and/or catalysts in the synthesis of polyhydroxy based allyl ethers often results in high raw material cost.
Due to their oxidation ability to form a crosslinked coating at ambient condition without leading to yellowing, allyl ether compounds have been either incorporated into the coating resins as chemical building blocks or blended physically with unsaturated polymers.
The general oxidation chemistry of allyl ether compounds is well known and can be found in various literature, for instance, "Allyl ethers in solventless and water-based coatings", Journal of the Oil and Color Chemists' Association, Vol. 148 (II), 1025 (1965); "Effects of chemical structure of allyl ethers on polymerization and properties of multi-functional acrylate systems", Journal of the Applied Polymer Science, 42, 2681 (1991), "Influence of allyl ethers in coating resins", Journal of Polymer Science, Polymer Chemistry Ed., 29, 1638 (1991).
Numerous prior literature reveals the chemical incorporation of allyl ether compounds into polyesters or alkyds ("Air-drying oil free polyether-esters", Paint Technology, Apr. 17, 1962: U.S. Pat. Nos. 4,670,308; 4,745,141; 4,997,480; European Patent Application 0234641), polyurethanes (U.S. Pat. No. 4,760,111, and European Patent Application 0315920), reactive diluent (U.S. Pat. No. 4,091,052 and UK Patent Application GB 2,190,672) and epoxy ("Surface coatings based on water-soluble epoxy allyl ether polymers systems", Journal of the Applied Polymer Science, 32, 3177 (1986). However, all the allyl ether compounds used as chemical building blocks in the literature were polyhydroxy based allyl ethers.
The following three patents disclose a certain allyl ether alcohol compound as a useful compound to prepare certain specific coating compositions, but not to the preparation of alkyd compositions.
U.S. Pat. Nos. 4,156,667 and 4,195,102 reveal an allyl ether alcohol dispersing solvent for an aqueous polyester and aminoplast system. A baking process at 120.degree.-130.degree. C. was employed to incorporate allyl ether alcohol into a coating composition through the reaction with aminoplast resin. Since allyl ether alcohol is physically blended with a coating material, purge was required prior to application to minimize volatility and toxicity problems. No air-drying coating composition with chemically bound allyloxypropanol, i.e. O-allyl polypropylene glycol, is mentioned.
U.S. Pat. No. 4,005,041 disclosed air-drying or heat curable polyurethane coating compositions prepared with allyl ether hydroxy compounds. Although allyl oxypropanol is suggested as a possible allyl ether hydroxy compound, all the examples were limited to trimethylolpropane diallyl ether (TMPDAE). Since strong hydrogen bonding in polyurethanes contributes significantly to improving their physical properties, polyesters and alkyds need a different molecular structure in order to achieve desired coating properties. This structure becomes more important when a monofunctional allyl oxypropanol is employed in place of a multi-functional allyl compound such as trimethylolpropane diallyl ether (TMPDAE) as it usually provides better dry and hardness because of its high crosslinking density in a cured film.
In addition, the air-drying polyurethane compositions in U.S. Pat. No. 4,005,041, includes no air-drying oil or fatty acid. The addition of castor oil, which mostly consists of non-drying fatty acid, was suggested as a plasticizer in order to improve flexibility. Therefore, the present invention combining an air-drying fatty acid (oil) and allylether compound is not suggested by U.S. Pat. No. 4,005,041 in which only air-drying polyurethanes using an allylether is described.
In summary, none of the above prior art publications reveal an air-drying alkyd composition or structure including chemically bound an allyloxy propanol.
Due to their oxidation ability to form a crosslinked coating at ambient condition without leading to yellowing, the allyl ether compounds have been either incorporated into the coating resins as chemical building blocks or blended physically with unsaturated polymers.